1. Field of the Invention
The present invention generally relates to an air-fuel ratio control apparatus for an internal combustion engine installed on an automobile or a motor vehicle. More particularly, the present invention is concerned with a technique for improving or enhancing an acceleration performance of the internal combustion engine equipped with the air-fuel ratio control apparatus which incorporates therein an air-fuel ratio feedback control function and a purge control function.
2. Description of Related Art
In general, the air-fuel ratio control apparatus for the internal combustion engine ordinarily incorporates the purge control function for causing a fuel vapor (i.e., vaporized fuel) originating in a fuel tank or the like to be adsorbed by activated carbon and purged to be introduced into an intake system of the engine as occasion arises. Further, the fuel injection apparatus of the internal combustion engine is equipped with an air-fuel ratio feedback control function for making the air-fuel ratio of the air-fuel mixture coincide with the theoretical air-fuel ratio.
In the air-fuel ratio control apparatus for the internal combustion engine equipped with the air-fuel ratio feedback control function and the purge control function as described above, the air-fuel ratio feedback correcting coefficient (multiplication coefficient) changes around a reference value (e.g. 1.0) when the adsorbed fuel vapor is not undergoing the purge process.
On the other hand, when the purge process is started, the fuel injection quantity has to be decreased by an amount or quantity corresponding to that of the purged fuel vapor introduced into the intake system. Accordingly, the air-fuel ratio feedback correcting coefficient is set to a value smaller than 1.0.
In that case, deviation or difference between the air-fuel ratio feedback correcting coefficient (<1.0) and the reference value (=1.0) when the purge process is being effected and the reference value (=1.0) assumes a variable value in dependence on the operation state of the internal combustion engine, i.e., ratio between the purge quantity and the intake air quantity (hereinafter referred to as the purge ratio).
Further, the air-fuel ratio feedback correcting coefficient is so determined as to change relatively slowly in accordance with a predetermined integration constant with a view to evading a sudden change of the air-fuel ratio.
Consequently, when the purge ratio changes in the course of the purge process due to transient operation, relatively much time is taken for the purge ratio to settle at the value after the change from the preceding value. Consequently, it is impossible to maintain the air-fuel ratio at the theoretical air-fuel ratio (=14.7) during a time period taken for the purge ratio to become steady.
Under the circumstances, there has been proposed an air-fuel ratio control apparatus for the internal combustion engine which apparatus is designed to make the air-fuel ratio feedback correcting coefficient coincide with a desired value by correcting the fuel injection quantity in accordance with the purge air concentration correcting coefficient during the purge process. In this conjunction, reference may have to be made to, for example, Japanese Patent Application Laid-Open Publication No. 261038/1996 (JP-A-1996-261038).
In the air-fuel ratio control apparatus mentioned above, the purge ratio is arithmetically determined or computed on the basis of the engine operation state and the purge quantity, a purge air concentration is computed on the basis of the purge ratio and the air-fuel ratio feedback correcting coefficient, a purge air concentration correcting coefficient is computed on the basis of the purge ratio and the purge air concentration, and then the fuel injection quantity is corrected in conformance with the purge air concentration correcting coefficient to thereby effectuate the control for making the air-fuel ratio feedback correcting coefficient coincide with a target or desired value.
In this conjunction, it is noted that when the internal combustion engine is accelerated with the purge air being introduced to the engine, vacuum or negative pressure (absolute value) within the intake passage decreases while the intake quantity increases. Besides, the purge air concentration of the intake air decreases remarkably in accompanying the decrease of the adsorbed fuel. Accordingly, there arises the necessity of controlling the air-fuel ratio toward richness of the air-fuel mixture by increasing the fuel injection quantity.
However, in the case where the purge air concentration and the purge air concentration correcting coefficient are computed on the basis of the purge ratio as mentioned above, the purge air concentration correcting coefficient updated to a value smaller than 1.0 by learning the immediately preceding engine operation state will gradually increase (approach to 1.0) in response to lowering of the purge ratio when the engine is accelerated, as a result of which the air-fuel ratio changes toward richness of the air-fuel mixture.
The air-fuel ratio control apparatus for the internal combustion engine known heretofore suffers a problem that even when the fuel injection quantity is corrected with the purge air concentration arithmetically determined from the purge ratio and the air-fuel ratio feedback correcting coefficient so that the air-fuel ratio feedback correcting coefficient becomes constant, the purge air of high purge ratio (i.e., remarkably rich purge air) will unwantedly be introduced in the intake system of the engine because it takes a lot of time for the purge air concentration correcting coefficient to be updated to a value for enriching the air-fuel mixture in response to lowering of the purge ratio for the enriching demand upon acceleration of the engine.
In particular, in the case where the engine operation is suddenly accelerated in the state where the purge air concentration correcting coefficient has been updated to a value for remarkably leaning the air-fuel mixture (i.e., value sufficiently smaller than 1.0 and closer to zero) due to the rich purge air in the initial phase, the air-fuel ratio remains on the lean side over a long time period taken for the purge air concentration correcting coefficient to assume the enriching value (i.e., to resume the value of 1.0), as a result of which degradation of the acceleration performance such as hesitation will unwantedly be incurred.